Anneal for magnesium alloys



United States Patent 3,039,901 ANNEAL FOR MAGNESIUM ALLOYS Guy F. Brackett, Jr., Florissant, M0., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware N0 Drawing. Filed July 9, 1959, Ser. No. 825,888 Claims. (Cl. 14820.3)

This invention relates to an improved annealing process for rolled magnesium based alloys containing thorium. It more particularly relates to heat treating rolled magnesium based thorium containing alloys by including water vapor in the heat treating atmosphere, the addition of which gas markedly facilitates the ease of cleaning of the final heat treated product.

The production of clean, smooth surfaced magnesium thorium sheet alloys such as the ASTM designated HK31 (nominal composition: 3 percent thorium, 0.7 percent zirconium, balance magnesium) or HM21 (nominal composition: 2 percent thorium, 1 percent manganese, balance magnesium) normally is achieved only with difficulty.

In the production of magnesium-thorium alloy sheet, the alloy normally is heat treated above 700 Fahrenheit in order to facilitate hot rolling of the sheet. At such temperatures the roll pickup contaminant, which appears on the final rolled sheet as embedded saltand pepperlike particles, becomes insoluble in the acetic acidsodium nitrate pickle (H. K. Delong, US. Patent 2,607,-

739) and other acid pickles normally used for cleaning magnesium alloys. For this reason clean magnesiumthorium rolled sheet can be obtained only by use of drastic cleaning treatments which lead both to excessive metal losses and the production of rough, etched, and pitted sheet. The problems involved in removing the roll pickup contaminant from the magnesium-thorium alloys now unexpectedly have been overcome in accordance with this invention by adding at least 10 percent water vapor by volume to the furnace atmosphere during the high temperature (i.e., above 700 Fahrenheit) heat treatment step. The presence of this water vapor in the furnace atmosphere keeps the roll pickup in a form readily soluble in the normal production acid pickles.

It is the principal object of this invention, therefore, to provide a method for the high temperature heating of rolled magnesium based-thorium alloys so that the products can be cleaned easily and satisfactorily by use of the normal production pickles and cleaning methods. A further advantage of the method is to reduce the time and amount of handling involved in finishing magnesiumthorium alloy rolled sheet. A further object is to provide a finished magnesium-thorium sheet markedly smoother and more pleasing in appearance than obtained previously. Other objects and advantages will become apparent from the method description presented hereinafter.

Preliminary reduction of a magnesium-thorium alloy ingot to a predetermined thickness suitable for final hot rolhng is carried out in a conventional manner as on a reversing break-down mill. The subsequent hot rolling to the desired final sheet thickness is achieved by rolling and heat treating at temperatures above about 700 Fahrenheit between passes.

According to the method of this invention, the heat treating is performed in a conventional heat treating furnace in the presence of an atmosphere containing at least 10 percent water vapor by volume. Introduction of the water vapor into the atmosphere present in heat treating furnace can be achieved by a variety of means. For example, steam or water from external sources can 3,039,901 Patented June 19, 1962 be introduced into the furnace atmosphere during the heat treating, or, open-topped containers of water can be placed beside the metal inside the furnace. In addition, the magnesium alloy sheet can, during the heating within the furnace, be enclosed in a hood or covered and the water vapor be introduced directly into the so produced enclosure. This latter innovation is advantageous for it eases the control of the atmosphere in contact with the metal and reduces the total amount of water vapor necessary to feed into the furnace to obtain the desired results upon subsequent pickling of the rolled metal.

The following examples will serve further to illustrate the invention.

Example I An ingot of HK31 alloy 12 inches thick, 40 inches wide and 76 inches long was heated at 900 Fahrenheit for 16 hours in a gas fired oven. Subsequently, the ingot was rolled on a reversing break down mill in a conventional manner. In the first passes, the ingot was passed between rolls sideways to simultaneously widen and reduce the thickness of the ingot. Following these passes, the ingot was passed through the rolls endways using a sufiicient number of passes to produce a coiled sheet of about 0.300 inch thickness. The coil so prepared was subjected to the method of the present invention in the following manner. The resulting coil and an open container of water were placed in a gas fired furnace set at 900 Fahrenheit whereby water vapor was evaporated from the container into the ambient atmosphere of the furnace. In addition to the water vapor supplied by the water in the container, steam from an external source was supplied continuously during the A heat treating to give a furnace atmosphere approaching percent water vapor by volume.

The coil was heated to and maintained at about 800 Fahrenheit for one hour after which time it was allowed to cool, the water vapor addition being maintained until the metal had cooled below 700 Fahrenheit. The cooled coil then was removed from the oven, reheated to 650 Fahrenheit in a direct gas-fired oven, rolled from 0.300 inch to 0.245 inch in one pass and recoiled. This so-rolled metal coil again was annealed in the water vapor atmosphere as described above, cooled, reheated to 650 Fahrenheit and rolled to 0.195 inch in one pass and again recoiled. After this reduction, the coil was cooled to room temperature and wire brushed in one pass through the wire brush machine as is customary in normal production practice. The coil then was given a final heating (in an oven set at 900 Fahrenheit) to 850 Fahrenheit in the water vapor atmosphere, rapidly removed from the furnace, immediately rolled to 0.127 inch and recoiled.

This coil then was sheared to sheet lengths and pickled by conventional techniques with a total metal removal of .0015 to .0020 inch (normal for acetic acid-sodium nitrate production pickle). Inspection of the finished sheet showed the complete absence of residual roll pickup normally found on HK31 sheet which had received conventional high temperature processing. This present sheet also had a smooth surface markedly different from the etched and pitted product resulting from non-water vapor annealed, pickled, and hand salvaged sheet.

For comparative purposes, a second ingot of HK31 alloy was heat treated and hot rolled in a conventional manner, the in process high temperature (above 700 Fahrenheit) heating being carried out directly in the gas fired heat treating furnace. (Analysis of the atmosphere of this furnace indicated the presence of about 6 percent water vapor on a volume basis.)

The rolled sheet obtained in this case was given a normal production pickle. This pickle removed only the most loosely held insoluble surface contaminant, the majority of the embedded insoluble roll pickup remaining on the sheet as salt-and-pepper like particles. The bulk of these embedded particles were removed only after a lengthy pickle whereby these particles were undercut and lifted out of the sheet. The sheet after cleaning in this manner, had a distinctly etched, rough, and pitted appearance rendering it commercially unacceptable.

Example 11 A small laboratory electric furnace was heated to 950 Fahrenheit. Air saturated with water vapor at 50 centigrade, which air-water vapor mixture is calculated to contain approximately 13 percent water vapor on a volume basis, was passed into the furnace during the heating of the furnace and for an additional hour after the 950 Fahrenheit temperature had been reached before introduction of the magnesium-thorium alloy samples into the furnace. After this time, two /2 inch x 3 inches x 0.125 inch thick panels of HK31 alloy, cut from an unclean rolled sheet having roll pickup on the surfaces, were loosely clamped together and placed in the furnace. The panels were clamped together to simulate a rolled coil or stack of flat sheets as is normally encountered in production practices. The panels were held in the atmosphere of airl3 percent water vapor for 6 hours at 950 Fahrenheit, after which time they were removed from the furnace, cooled and cleaned with the conventional acetic acid-sodium nitrate pickle. This pickling gave essentially completely clean, smooth surfaced panels.

Example III Treating two similarly roll pickup contaminated panels in much the same manner as Example II but in an atmosphere of air saturated with water vapor at 75 centigrade (mixture calculated to contain approximately 50 percent water vapor on a volume basis) gave essentially completely clean smooth surfaced panels when the so-heat treated panels were subjected to the conventional acetic acid-sodium nitrate pickle.

Example 1V Two similarly roll pickup contaminated panels were subjected to heating in the electric furnace the same as in Example II except steam was introduced directly into the furnace during the heating to produce an atmosphere containing 100 percent water vapor by volume.

Completely clean, smooth surfaced panels were obtained when the so heat treated samples were subjected to the conventional acetic acid-sodium nitrate pickle.

While heat treating atmospheres containing a minimum of about percent water vapor by volume have been shown to be effective in maintaining the roll pickup in a pickle soluble state, desirably the atmosphere should contain at least percent water vapor by volume, and preferably should contain at least percent water vapor by volume.

Various modifications can be made in the present invention without departing from the spirit or scope thereof, for it is to be understood that I limit myself only as 7 v in anatmosphe re containing by volume at least 10 per- 7 cent water vapor. r

2. A process for heating a rolled product of a magnesium alloy contain'ingfrom about 0.5 to about 8 percent thorium, the surface contaminants of which product become insoluble above 700 Fahrenheit, the improved method of heat treating whichcomprisesheating the rolled product at temperatures from about 700 to about l000 Fahrenheit in an atmosphere containing by volume at least 10 percent water vapor, said heating process being carried out at about normal atmospheric pressure and for a time sufficient to facilitate the subsequent hot rolling of said metal.

3. A process for heating a rolled product of a magnesium alloy containing from about 0.5 to about 8 percent thorium, the surface contaminants of which product become insoluble on heating above 700 Fahrenheit, the improved method of heat treating which comprises heating the rolled product at temperatures from about 700 to about 1000 Fahrenheit in an atmosphere containing by volume about 20 percent water vapor, said heating being carried out at about normal atmospheric pressure and for a time sufiicient to facilitate the subsequent hot rolling of said metal.

4. A process for heating a rolled product of a magnesium alloy containing from about 0.5 to about 8 percent thorium, the surface contaminants of which product become insoluble above 700 Fahrenheit, the improved method of heat treating which comprises heating the rolled product at temperatures from about 700 to about 1000 Fahrenheit in an atmosphere of water vapor, said heating being carried out at about normal atmospheric pressure and for a time suflicient to facilitate the subsequent hot rolling of said metal.

5. A process for heating a rolled product of HK31 alloy, the surface contaminants of which product become insoluble above 700 Fahrenheit, the improved method of heat treating which comprises heating the rolled product at temperatures from about 700 to about 1000" Fahrenheit in an atmosphere containing by volume at least 10 percent water vapor, said heating process being carried out at about normal atmospheric pressure and for a time suflicient to facilitate the subsequent hot rolling of said metal.

6. A process for heating a rolled product of HM21 alloy, the surface contaminants of which product become insoluble above 700 Fahrenheit, the improved method of heat treating which comprises heating the rolled product at temperatures from about 700 to about 1000 Fahrenheit in an atmosphere containing by volume at least 10 percent water vapor, said heating being carried out at about normal atmospheric pressure and for a time sufiicient to facilitate the subsequent hot rolling of said metal.

7. A process for heating a rolled product of HK31 alloy, the surface contaminants of which product become insoluble above 700 Fahrenheit, the improved method of heat treating which comprises heating the rolled product at temperatures from about 700 to about 1000" Fahrenheit in an atmosphere containing by volume about 20 percent water vapor, said heating being carried out at about normal atmospheric pressure and for a time sufiicient to facilitate the subsequent hot rolling of said metal.

8. A process for heating a rolled product of HMZI alloy, the surface contaminants of which product become insoluble above 700 Fahrenheit, the improved method of heat treating which comprises heating the rolled product at temperatures from about 700 to about 1000* Fahrenheit in an atmosphere containing by volume about 20 percent water vapor, said heating being carried out at about normal atmospheric pressure and for a time suflicient to facilitate the subsequent hot rolling of said metal.

9. A process for heating a rolled product of HK3l alloy, the surface contaminants of which product become insoluble above 700 Fahrenheit, the improved method of heat treating which comprises heating the rolled produ ct at about 900 Fahrenheit in an atmosphere of percent water vapor by. volume, said heating being carried out at about'normal atmospheric pressure and for a time sufficient to facilitate the subsequent hot rolling of said metal.

10. A process for heating 9. rolled product of HM21 alloy, the surface contaminants of Which product become insoluble above 700 Fahrenheit, the improved method of heat treating which comprises heating the rolled prodnot at about 900 Fahrenheit in an atmosphere of 100 percent water vapor by volume, said heating being carried out at about normal atmospheric pressure and for a time suflicient to facilitate the subsequent hot rolling of said metal.

References Cited in 216 file of this patent UNITED STATES PATENTS Keller Apr. 2, 1935 Arditi Nov. 28, 1950 OTHER REFERENCES The Physical Metallurgy of Magnesium and Its Alloys, by Raynor, Pergamon Press, 1959 (p. 473 relied upon). 

1. A PROCESS FOR HEATING A ROLLED PRODUCT OF A MAGNESIUM BASED THORIUM CONTAINING ALLOY, THE SURFACE CONTAMINANTS OF WHICH BECOME INSOLUBLE ABOVE 700*FAHRENHEIT, THE IMPROVED METHOD OF HEAT TREATING WHICH COMPRISES HEATING THE ROLLED PRODUCT ABOVE 700*FAHRENHEIT IN AN ATMOSPHERE CONTAINING BY VOLUME AT LEAST 10 PERCENT WATER VAPOR. 